Process for refining pig iron

ABSTRACT

Process for refining pig iron with technically pure oxygen within a converter or similar receptacle wherein the oxygen is supplied to the iron bath simultaneously from above and as well as from below, characterized in that oxygen is supplied to the bath from above by blowing onto the bath surface and in that the oxygen supplied to the bath from below is supplied by an oxygen jet in the proximity thereof hydrocarbons, particularly hydrocarbons of higher molecular weight, are introduced into the bath in a gaseous or liquid condition.

The invention refers to a process for refining pig iron with technicallypure oxygen within a converter or similar receptacle, wherein the oxygenis supplied to the iron bath simultaneously from above and from below.The invention further refers to an apparatus for performing thisprocess.

It is known that the process for blowing oxygen onto the surface of aniron bath and having become known as the so-called LD-process permittedfor the first time the use of technically pure oxygen for refining pigiron and permitted high additions of scrap. Therefore, this process soonencountered wide-spread use.

When blowing oxygen onto the surface of the iron bath the oxygenimpinging the bath surface will vigorously react with the carbon presentwithin the bath, which results in a deep-reaching bath agitation. Inview of this bath agitation still new portions of the bath are broughtup to the surface thereof, thus maintaining the refining reaction. Withthe carbon content of the bath becoming reduced to 0.2 to 0.05 percentby weight the reaction of the oxygen with the reduced amount of carbonwill become weaker, which will be the case also with the bath agitation.The composition of the bath at different height levels will also becomeless uniform and the reaction will slow down so that optimum rate ofdecarburization can no more be maintained. When producing soft steels,this will result in longer refining periods and in a relatively highcontent in iron oxides within the slag. A further disadvantage of thisprocess has been seen in the reddish brown fumes produced which requireddust separation from the waste gases.

For these reasons many proposals have been made to avoid the mentioneddrawbacks of the process mentioned. A known process (US-PS 3,030,203)resides in interrupting oxygen supply onto the bath surface whenattaining a carbon content between from 0.20 to 0.05 percent by weight,in then tilting the converter into approximately a horizontal positionand then to complete refining of the melt by means of an oxygen lanceintroduced into the melt in an inclined position. This process was notin practical use, obviously in view of the fact that when refining themelt by means of a submerged lance within a tilted converter the bathsurface is of considerable extent and the resulting reaction will stillbe confined to a locally limited area having as a consequence prolongedrefining periods.

For avoiding the reddish brown fumes there has been proposed (GB-PS882,676) to blow the oxygen into the bath from above by means of aconsumable coaxial tube lance, noting that the oxygen is supplied viathe inner tube whereas an other combustible gas, f.i. hydrocarbons, isblown into the bath via the annular space between the inner tube and theouter tube, so that any mixing of the gases may take place only outsideof the lance mouth. The proportions of the gases are selected such thatthere is an excess of oxygen. This measure shall suppress the formationof the reddish brown fumes without exerting any excessive cooling actionon the bath. Also this proposal was not used in practice becauseformation of reddish brown fumes can only partially be reduced so thatthe waste gases have still to be subjected to a dust removal step. Thisproposal however perhaps gave rise to the further proposal (FR-PS1,450,718) to supply the oxygen according to the mentioned manner bymeans of a coaxial tube through the converter bottom, i.e. from below,into the melt, noting that also in this case the oxygen is supplied viathe inner tube and hydrocarbon is supplied via the annular space betweenthe inner tube and the outer tube as an additional gas or sheathing gas.Also these measures did not result in a complete suppression of theformation of the reddish brown fumes within the waste gases but resultedin a good turbulence of the bath and, for the first time, in anacceptable service life of the converter bottom and of the nozzles forsupplying pure oxygen and additional gas, so that this proposal hasalready been used in practice. This process shows essential drawbacksconsisting in that the bath will become enriched in hydrogen as soon asthe carbon content of the bath becomes low and the formation of carbonmonoxide becomes reduced and that this hydrogen has to be removed byadditional operation steps, in that also the waste gases have a highcontent in hydrogen which is disturbing in the dust removal step, and inthat the slag floating on the bath is relatively cold and will easilysolidify so that a high amount of ejections is to be taken into account.Ejections of material can only be avoided by introducing lime togetherwith a stream of oxygen which will result in a high expenditure in viewof the increased number of nozzles required.

For ensuring the required turbulence of the bath and for suppressing thereddish brown fumes it has also been proposed (US-PS 3,259,484) tosupply oxygen to the bath simultaneously from above and from below,noting that the major portion of the oxygen required for refining actionis to be supplied through the pores of a porous converter bottom. Bystrongly subdividing the oxygen stream it is intended to avoid localsuperheating so that formation of reddish brown fumes will be suppressedand a good service life of the converter bottom will be guaranteed. Theoxygen supplied into the bath from above shall be loaded, according tothis proposal, with lime required for the slag formation and be suppliedby means of a lance submerged into the bath. By cooling the area ofreaction with lime and by positioning the area of reaction at a locationbelow the bath surface, the formation of reddish brown fumes shall alsobe avoided. This proposal has not found practical use, obviously mainlybecause no acceptable service life of the converter bottom could beattained and the formation of reddish brown fumes could, as is the casewith all refining processes operated with pure oxygen, not be suppressedto such an extent that subsequent dust removal from the waste gasescould be avoided.

It is an object of the invention to avoid the mentioned drawbacks as faras they can be avoided. In processes as described in the beginning andin processes as last mentioned, this is achieved by supplying the oxygento the bath from above by impingement from above and by supplying theoxygen to the bath from below by means of a jet adjacent of which areintroduced into the bath hydrocarbons, particularly hydrocarbons ofhigher molecular weight, in a gaseous or liquid condition. The inventionaccordingly consists in a combination of two features known per se and,respectively, processes known per se, both of which have been used inpractice but have up till now, been considered as competing processes.Preferably the oxygen jet supplied from below is, in a manner known perse, sheathed by a sheath of gaseous hydrocarbon by using a sheathingtube (coaxial tubes).

By the invention, dust removal from the waste gases does not becomesuperfluous but notwithstanding many advantages result as compared withthe known processes. Compared with processes simply supplying oxygenfrom above, the refining periods are shortened, the amount of addedscrap or ore can be increased and in view of the increased turbulencethe content of the slag in iron oxides is reduced when producing lowcarbon steels. As compared with the processes simply operated withsheathing tubes (coaxial tubes) supplying the gases from below throughthe bottom of the converter, the formation of the slag is improved bythe hot bath surface and a more simple possibility for loading theoxygen with lime, this because the oxygen itself can be loaded withlime, is provided. In connection therewith the converter ejections arereduced and the yield is increased. Furthermore, the waste gas has alower content in hydrogen and the danger of an increased content inhydrogen of the steel produced is not present because, in principle,less gaseous hydrocarbons are required and the hydrogen absorbed fromthe bottom nozzles is washed out by the pure carbon monoxide produced onrefining the bath by supplying oxygen from above. Furthermore, theexpenditure for the oxygen lance is approximately compensated by thereduced number of required bottom nozzles, the required fittingsincluded. With smaller converters one can even do with one single bottomnozzle. In view of the refining being effected for the major part fromabove the bath and thus requiring less bottom nozzles and, respectively,a shorter time for fully operating the bottom nozzles, and,respectively, a reduced gas supply to the nozzles, it is furtherpossible to easily match the service life of the converter bottom to theservice life of the other converter lining without the necessity to makethe converter bottom excessively thick. Therefore, according to afurther feature of the invention, it is proposed to select the pressureof the oxygen supplied from below at the beginning of the refiningprocess so that the pig iron is only prevented from entering into thenozzles, and to increase the pressure only during processing.

The invention further refers to an apparatus for performing the processaccording to the invention and consisting of a converter and an oxygenlance for blowing oxygen from above and adapted to be lowered from abovethrough the converter mouth, noting that the converter bottom comprisesone or more sheathing gas nozzles (coaxial tubes) for supplying oxygenat the one hand and gaseous hydrocarbons as sheathing gas on the otherhand.

An embodiment of the invention will be described with reference to thedrawing.

The drawing is illustrating an apparatus according to the invention. Thereceptacle, a converter 1, is provided with an oxygen lance 2 forsupplying oxygen from above and arranged for being lowered through theconverter mouth 3. The converter bottom 4 comprises a bottom nozzle 5,consisting of two concentric tubes 8 and 9 fitted one into the other.The inner tube 8 is for supplying oxygen, whereas a gaseous hydrocarbon,f.i. natural gas, is supplied via the annular space between inner tube 8and outer tube 9. With greater converters a plurality of bottom nozzles5 are provided. The process is preferably as follows: First, theconverter 1 is tilted for a small angle and charged with scrap.Subsequently, nitrogen is introduced into the converter via both supplyconduits 10, 11 and pig iron 6 is charged into the converter, whereuponthe converter is again put in upright position. The oxygen lance 2 isthen lowered through the converter mouth 3 and oxygen loaded with limedust is blown onto the pig iron. Subsequently, by switching over valves(not shown) provided in the supply conduits 10, 11 leading to nozzletubes 8 and 9 oxygen is supplied into nozzle tube 8 and natural gas issupplied into nozzle tube 9. The pressure of both gases is firstadjusted so that the pig iron is reliably prevented from entering thenozzles. In view of the high temperature of the bath surface a slaglayer 7 is formed from the lime dust, and the pig iron is being rapidlyrefined. As soon as the refining action becomes reduced, the pressure ofthe oxygen within the supply conduit 11 leading to the tube 8 of thebottom nozzle 5 is increased and the pressure within the supply conduitleading to oxygen lance 2 is somewhat reduced so that the proportion ofthe refining action exerted by the oxygen supplied via the bottom nozzle5 is increased and will attain about 50 percent of the total refiningaction. As soon as the desired carbon content has been reached, thebottom nozzle 5 is again supplied with nitrogen and the oxygen lance 2is lifted and thereby stopping oxygen supply to said lance 2.Subsequently the refined melt may be tapped into a pouring vesseltogether with required additions.

It is obvious to the man skilled in the art that modifications andvariations of the embodiment are possible, if required. The slag may betapped off during refining and provision can be made for temporarilysupplying to the bottom nozzle 5 other gases, f.i. argon, for degassingpurposes or other metallurgical effects.

The hydrocarbons used as a sheathing gas may be those as derived frompetroleum, particularly those having in their molecules up to 20 carbonatoms; natural gas is the most preferred hydrocarbon because of its easeof availability, fuel oil may, however, also be used because it will becracked under the influence of the high process temperature to a gascontaining components of low molecular weight.

What I claim is:
 1. In a process for refining pig iron with technicallypure oxygen within a converter wherein the oxygen is supplied to thebath simultaneously from above and from below the surface of the melt byimpingement onto the bath surface by means of a lance and by means of atleast one bottom nozzle below the surface of the bath, said nozzleproviding an oxygen jet, said nozzle also providing hydrocarbons as asheathing for said oxygen jet, the improvement comprising undertakingsaid refining by impinging oxygen onto the surface of the bath whileadjusting the pressure of the oxygen supplied from below at thebeginning of the refining process so that the pressure is adequate toprevent said pig iron from entering said nozzle; continuing saidrefining by impinging until the amount of carbon in the bath contents isbetween 0.2 and 0.05 percent by weight; and increasing the pressure ofoxygen introduced from below when the amount of carbon in the bathcontent is between 0.2 and 0.05 percent by weight, thereby enhancing theturbulence and stirring action and the rate of refining action whichdecrease when the amount of carbon in the bath content falls to between0.2 and 0.05 percent by weight.
 2. A process as in claim 1 furthercomprising introducing nitrogen through the bottom nozzle during thecharging of said converter.
 3. A process as in claim 1 furthercomprising reducing the oxygen supply to the lance as soon as therefining action slows down.
 4. A process as in claim 1 furthercomprising introducing nitrogen through the bottom nozzle during thecharging of the converter.